Mail routing

[#1]

I have a .co.uk domain that's been hosted by PlusNet for a year or more. The domain's "A" record points to the PlusNet (force9) nameservers, so web requests are correctly served by the PlusNet web hosting. Mail to that domain also finds its way to my mailboxes at PlusNet too, but as I already had lots of routing rules set up elsewhere (at my registrar - 123-reg) I wanted to continue using them. Following advice in here I set the domain's MX records to point at the 123-reg servers.
All has been working fine since I set it up, but I recently noticed that some mail to mydomain.co.uk was getting unexpectedly bounced.
I /think/ the problem may have arisen about a month ago when I changed my PlusNet spam filtering settings. Prior to that when my external routing rules sent e.g. foo@mydomain.co.uk -> me@myaccount.plusnet.com it arrived in my appropriate PlusNet mailbox. Now it seems that such mails will be bounced unless foo is set up as an alias in my PlusNet mail settings.
How can I set things back to the way they were, with all mails that find their way to me@myaccount.plusnet.com being accepted regardless of who they were originally addressed to?

[#2]

Is the 'Catch all' switched off?

[#3]

Yes, the CatchAll is off but that's the way I want it and AFAIAA that's the way it was before things started to go awry. I have just a few PlusNet mailboxes, and my external mail routing directs stuff to one of them (computing@mydomain.co.uk -> him@myaccount.plusnet, gardening@mydomain -> her@myaccount.plusnet, that sort of thing). If I don't have a specific rule defined for something@mydomain then I don't want it.

[#4]

OK, now working again. I turned off spam filtering on mydomain and all seems to be back to the way it was, it's just that it took about 24hrs for the change to take effect. ISTR seeing a warning that it can take a while for spam filtering changes to take effect, to be fair (why so long though?). What I hadn't initially expected is that applying spam filtering to a hosted domain would trigger all the alias filtering too. Lesson learned!
Although I now have no spam filtering on mydomain, I do have it enabled on myaccount.plusnet.com. Since all main to mydomain is forwarded to myaccount.plusnet.com, can I assume that it will be subject to spam filtering there?

[#5]

Quote from: Boxersoft

OK, now working again. I turned off spam filtering on mydomain and all seems to be back to the way it was, it's just that it took about 24hrs for the change to take effect. ISTR seeing a warning that it can take a while for spam filtering changes to take effect, to be fair (why so long though?).

Because it's a DNS change and DNS takes time to propagate. Switching spam filtering off basically changes the servers your mail is routed to, from:

MX preference = 10, mail exchanger = mx.avasin.plus.net
mx.avasin.plus.net      internet address = 212.159.8.200
mx.avasin.plus.net      internet address = 212.159.9.200

Which are our anti-spam appliances, to:

MX preference = 10, mail exchanger = mx.core.plus.net
mx.core.plus.net        internet address = 212.159.9.1
mx.core.plus.net        internet address = 212.159.8.1

Which are our core delivery servers.
Looking at your domain records, you're pointing your mail at both 123-reg *and* Plusnet:

MX preference = 8, mail exchanger = mx1.123-reg.co.uk
MX preference = 5, mail exchanger = mx0.123-reg.co.uk
MX preference = 10, mail exchanger = mx.core.plus.net

With things set up like this, I'd warrant a guess that a fair bit of spam will end up getting delivered directly to your Plusnet mailbox without going anywhere near 123-reg.
To get rid of the Plusnet records you need to specify 'Other' under the DNS settings for your domain:

I suspect it's this that's been the real cause of your problem rather than the configuration of the spam filter.

[#6]

Thanks Bob, that's very helpful. I'll have a go at sorting out the settings this evening.
You're right about spam arriving with things as they are currently configured - they've been trickling in throughout the day.